1. Brief Description of the Invention
This invention generally addresses needs in the inerting of molten or solid metals, and in particular methods and apparatus to improve efficiency of use of cryogens such as argon in inerting molten or solid metals.
2. Related Art
In the metal casting industry, metals (ferrous or non-ferrous) are melted in a furnace, then poured into molds to solidify into castings. In the foundry melting operations, metals are commonly melted in electric induction furnaces. It is often advantageous to melt the metals under cover of inert gas (usually Ar or N2), rather than expose the metal to atmospheric air. The inert gas cover minimizes oxidation of the metal (including its alloying components), which increases yield and alloy recovery efficiency, and also reduces formation of metallic oxides which can cause casting defects (inclusions). The inert gas cover also reduces the tendency of the molten metal to absorb gases (chiefly O2 and H2) from the atmosphere, which in turn reduces gas-related casting defects such as porosity. Other benefits of melt surface inerting include reduced slag formation, improved metal fluidity, increased furnace refractory life, and reduced need for de-oxidizers.
As the electric induction furnace is generally an open-top, batch melter, the inert gas (N2 or Ar) is usually applied from above the furnace. Inert gas is usually applied throughout the entire melting cycle.
There are many types of furnace inerting techniques in practice today, but they can generally be classified into two major categories: Gas inerting, in which gaseous N2 or Ar is (gently) blown into the top of the furnace; and liquid inerting, in which liquid N2 or Ar is dripped or poured into the top of the furnace. In gas inerting, there are many different configurations of pipes and manifolds or distribution xe2x80x9cringsxe2x80x9d employed to blow the inert gas into the top of the furnace. These make use of varying gas pressures, velocities, discharge locations and angles of injection. Some try to minimize turbulence by creating gentle laminar flow. Some utilize a xe2x80x9cswirlingxe2x80x9d pattern. Some techniques may employ a collar, shroud or cone-like assembly mounted on top of the furnace. However, with any gas inerting technique, it is difficult to produce and maintain a true inert (0% O2) atmosphere directly at the metal surface, because hot thermal updrafts from within the hot furnace are continually pushing the incoming cold inert gas up and away from the metal surface. As the hot air and gases rise, the induced draft is continually pulling fresh cold air toward the furnace. The injected inert gas will also entrain ambient air along with it as it is injected into the furnace. Because of these effects, it is difficult, if not impossible, for gas inerting techniques to provide a true inert (0% O2) atmosphere directly at the surface of the metal.
With liquid inerting (such as taught in U.S. Pat. No. 4,806,156), the liquid cryogen (typically N2 or Ar) has higher density than its gas phase and air, and is much less likely to be pushed up and away from the melt surface by the thermal updrafts. The liquid drops or stream are much better able to fall all the way down to the actual metal surface (hot solid metal or molten metal). After contacting the metal surface, within a short time, the liquid vaporizes into a gas. (The appearance is similar to drops of water xe2x80x9cdancingxe2x80x9d on a hot pancake griddle). As the N2 or Ar boils from liquid to gas, it expands volumetrically by a factor of 600-800 times as it rises. This expansion pushes ambient air away from the surface of the metal. In this manner, liquid inerting provides a more effective, true inert (0% O2) atmosphere directly at the metal surface, as compared to gas inerting. With liquid inerting, inert gas usage efficiency is generally increased; i.e. it requires a lower quantity of inert gas to achieve the same performance as gas inerting.
One drawback of liquid inerting is the difficulty of efficiently delivering the liquid N2 or Ar to the furnace interior in a liquid state. The liquefied gas (preferably N2 or Ar) is extremely cold (approximately xe2x88x92184xc2x0 C.). In the storage tank and distribution piping, the liquid inert gas is continually absorbing heat from the surroundings. This ambient heat pickup manifests itself by boiling some of the liquid to vapor inside the storage tank and distribution piping. The tank and piping is insulated as much as practically possible (typically 7 to 11 cm foam, or vacuum-jacket). The tank-to-furnace piping distance is kept as short as possible (in practice, usually about 15 to 50 m). In spite of these efforts, there is always some amount of liquid that will unavoidably boil to vapor, due to this ambient heat pickup. In addition, some liquid will always xe2x80x9cflashxe2x80x9d boil to vapor by virtue of pressure reduction alone. The liquid is stored at elevated pressure (typically 2 to 7 bar) in the storage tank, in equilibrium with its vapor phase. Elevated pressure is necessary to provide the driving force to xe2x80x9cpushxe2x80x9d the liquid out of the tank, through the distribution piping. As a matter of practicality, there is usually a vertical elevation rise in the piping which needs to be overcome, and there is some pressure drop through the final liquid discharge device (diffuser). So, as the liquid N2 or Ar travels through the piping, pressure decreases (eventually to atmospheric pressure at the discharge point), and more and more of the liquid boils to vapor. Due to these combined effects (ambient heat pickup and pressure drop), by the time the liquid N2 or Ar reaches the discharge point at the furnace, it is estimated that roughly 0.5% to as much as 30% has boiled to vapor, depending on the parameters of the particular system.
Due to volumetric expansion, the vapor (gas phase) occupies much more space than the liquid. In the piping, this expanding gas restricts, or xe2x80x9cchokesxe2x80x9d the flow of liquid by occupying a greater and greater portion of the volume available in the pipe. Hence the N2 or Ar in the pipe can be mostly liquid by mass, but mostly vapor by volume.
The result is that xe2x80x9csputteringxe2x80x9d or xe2x80x9csurgingxe2x80x9d flow is observed at the discharge end of the pipe. xe2x80x9cSputteringxe2x80x9d flow is a combination of gas and xe2x80x9csprayingxe2x80x9d liquid, often unsteady in appearance with time, with respect to the observed amount of liquid flow. xe2x80x9cSurgingxe2x80x9d flow is a more extreme condition, in that there is observed alternating time periods of xe2x80x9cgas onlyxe2x80x9d discharge, and xe2x80x9cgas plus liquid sputteringxe2x80x9d discharge. Sputtering and surging flow is caused by the generated vapor xe2x80x9cbubblesxe2x80x9d working their way out of the system piping. The greater the percentage of vaporization, the more extreme the observed sputtering and/or surging will be.
Sputtering or surging flow will reduce the furnace inerting effectiveness, for liquid inerting processes. Compared to a compact, well organized and steady (small) liquid stream, or compared to relatively large droplets, a spray or mist of fine liquid droplets will have much greater surface area, and will therefore absorb heat from the furnace environment much more quickly, vaporizing more quickly, and therefore be less likely to fall all the way down to the metal surface in the liquid state, therefore providing a less effective inert atmosphere at the metal surface. The most effective liquid inerting is provided by a compact, well-organized and steady liquid stream, or by a steady succession of relatively large liquid droplets (minimum liquid surface area).
It is common to use a diffuser, or tight mesh screen (typically sintered metal filter, approximately 40 micron size), at the discharge of the liquid pipe, in order to minimize sputtering flow. The diffuser xe2x80x9ccatchesxe2x80x9d the sputtering spray of gas and small liquid drops, reducing the liquid velocity and re-organizing the drops into larger liquid droplets or a steady liquid stream, which generally drips out the bottom portion of the diffuser, while the gas generally seeps out the top. This diffuser is surrounded by an outer shroud, or cone, which protects the diffuser from molten metal splash, and can also help to organize the emerging liquid droplets into a more focused, single stream. The diffuser/cone assembly, then, helps to provide a more compact, well-organized and steady liquid stream or succession of larger droplets, to improve furnace inerting effectiveness (i.e. reduces the percentage of emerging liquid droplets that evaporate in the furnace).
However, even by using a diffuser/cone assembly, and after minimizing the piping distance, insulating the pipe and tank as much as possible, and reducing the tank pressure to as low a level as possible, what emerges from the diffuser is still a combination of liquid and gas. Vaporization of some of the liquid to gas is unavoidable. In many cases, sputtering flow, or even surging flow is still observed. The greater the percentage of vapor mixed with gas, the more extreme the sputtering and/or surging will be, and the greater the reduction in furnace inerting effectiveness. As the ambient heat input to the system (tank plus piping) is relatively constant (function of total surface area and temperature difference), the absolute amount of liquid boiling to vapor will be essentially fixed, for a given system. So, as liquid flowrate is increased, the percentage of vapor will be reduced. In practice, then, in order to achieve a stable and consistent liquid flow, as opposed to surging, furnace operators will increase the total N2 or Ar (liquid) flow higher and higher until surging is eliminated (i.e. flood the system with liquid). In many cases, due to high levels of vaporization (caused by high ambient heat pickup or large pressure drop), the total N2 or Ar flowrate is higher than what it really needs to be, to provide effective and consistent inerting for a given furnace. This increases operating cost, and can create potential for explosions, by having too large a quantity of liquid pooling on top of the molten metal. Hence, in many cases, if vaporization could be reduced, then total flowrates could be reduced, and operating cost could be reduced while improving operator safety.
One way to reduce surging and to provide a more consistent, stable liquid flow is to remove the generated gas bubbles from the piping, prior to the diffuser. This is described in U.S. Pat. No. 4,848,751 (special lance). This method utilizes a double-wall (concentric) pipe as the last section of liquid Ar or N2 piping before the diffuser. A small hole is located in both the inner and outer pipes, pointing vertically upward, with the inner hole at the discharge end and the outer hole at the inlet end. Vapor generated inside the piping is allowed to escape through the inner hole. This cold xe2x80x9csacrificialxe2x80x9d vapor travels through the annular region between the inner and outer pipe, counter to the flow of liquid (and gas) in the inner pipe, and escapes to atmosphere through the hole in the outer pipe. Thus, some of the vapor is allowed to escape the piping system before the gas/liquid mixture discharges through the diffuser, and the escaping vapor is utilized to help cool (insulate from further heat pickup) the last section of pipe (generally positioned over the hot furnace) to reduce further evaporation. However, the chemical value of the escaping vapor is wasted, in that it is vented to atmosphere, rather than being sent into the furnace. Also, it is not clear that the size and location of the holes is optimum for each individual installation. Also, since gas bubbles generated in the piping generally will rise to the highest point in the piping system, it is not clear that all of the vapor will consistently and effectively be purged through these holes in the concentric pipe lance, since it is located at the discharge end of the piping (at the furnace), which is usually the lowest point in the system piping.
Another technique for removing vapor bubbles from the piping is to utilize a gas-liquid phase separator device in the piping. These are sometimes referred to as gas vents, or xe2x80x9ckeep-fullxe2x80x9d devices. These are commercially available devices. One or more are mounted, typically, in the highest point in the piping system, generally close to the discharge end. The gas vent device typically includes an internal float and valve mechanism, inside a small chamber. Liquid accumulates in the bottom of the chamber, raising the float by buoyancy force, which closes the gas vent valve on top of the chamber. As gas accumulates in the piping, generally rising to the highest point, it will accumulate in the xe2x80x9cdomexe2x80x9d or upper portion of the chamber, displacing liquid in the chamber, until eventually the float drops low enough to open the top gas vent valve. This allows gas to vent out the top, until enough liquid re-fills the bottom of the chamber to push the float up, which closes the vent valve. The cycle then repeats, indefinitely. This simple mechanical device helps to continually and automatically vent gas from the system piping, which increases the percentage of liquid, which helps to reduce surging flow, and allows the operator to reduce the total liquid flow required to maintain stable, consistent liquid flow. However, the purged as is vented to atmosphere, and again, its chemical (inerting) value is wasted. Also, while the gas vent valve periodically opens to vent gas to atmosphere, system pressure is reduced. This can reduce the driving force for pushing liquid through the piping system, causing the operator to increase tank pressure, which results in additional flash vaporization. Or, the periodic venting to atmosphere with subsequent pressure reduction can cause additional liquid to vaporize, due to the premature reduction in pressure.
Finally, in many liquid inerting systems, a small metering orifice is placed just upstream from the diffuser. This is sized to allow a constant xe2x80x9ccorrectxe2x80x9d amount of flow to the diffuser downstream. However, this can compound the severity of observed surging, since all liquid and gas must pass through this small orificexe2x80x94it can take longer for gas bubbles to work their way through this orifice. Also, with time, as system insulation value deteriorates, and heat input to the system increases, the percent vaporization increases, and the metering orifice may in fact become xe2x80x9ctoo smallxe2x80x9d, and surging flow is exhibited where at one time it was steady. The operator then is forced to increase the size of the metering orifice (open up the metering orifice valve).
It would be an advance in the art if more efficient methods and apparatus were developed to overcome some or all of the above problems.
In accordance with the present invention, methods and apparatus are presented which overcome some or all of the mentioned disadvantages of previous systems.
One aspect of the invention is an apparatus for efficient utilization of a cryogen in inerting of molten or solid metals, the apparatus comprising:
a) a source of liquid cryogen;
b) a conduit connected to said source of liquid cryogen for transporting said liquid cryogen to a gas/liquid separator (also denoted as a xe2x80x9cgas vent devicexe2x80x9d herein);
c) a first conduit connecting the gas/liquid separator to a cryogen inerting nozzle and adapted to supply liquid cryogen to the cryogen inerting nozzle, the cryogen inerting nozzle positioned over molten or solid metal in a container; and
d) a second conduit connecting the gas/liquid separator to the container at a position over the molten or solid metal, the second conduit adapted to supply gaseous cryogen to the molten or solid metal.
Preferably, the second conduit connects the gas/liquid separator to an outer section of the cryogen inerting nozzle, as further described herein. Preferred apparatus also comprise insulation for the first and second conduits to maintain temperature as low as possible.
A second aspect of the invention is a method for efficient utilization of a cryogen in inerting of solid or molten metals, the method comprising:
a) providing a source of liquid cryogen;
b) transporting said liquid cryogen through a conduit connected to the source of liquid cryogen to a gas/liquid separator (also denoted a gas vent device herein), wherein a portion of the liquid cryogen transforms into gaseous cryogen;
c) transporting a portion of the liquid cryogen through a first conduit connecting the gas/liquid separator to a cryogen supply nozzle positioned over solid or molten metal in a container;
d) transporting at least a portion of said gaseous cryogen through a second conduit connecting the gas/liquid separator to the container, and preferably to the cryogen inerting nozzle; and
e) flowing the portion of liquid cryogen through the cryogen inerting nozzle near a surface of solid or molten metal in the container, and flowing at least a portion of the gaseous cryogen near the same surface of solid or molten metal, preferably through the cryogen inerting nozzle.
Liquid inerting effectiveness can be decreased due to vaporization of liquid in the system piping, as described. Removal of this gas before reaching the discharge diffuser can improve inerting effectiveness. The new method utilizes this vented gas in the furnace to assist with inerting, rather than venting the gas to atmosphere. The known diffuser/cone assembly is replaced with a novel cryogen supply nozzle, preferably shaped like a cone, which incorporates a second connection for this gas. The gas vented from the piping (via a gas vent device) is fed to this novel cryogen supply nozzle. Preferably, a pressure regulator or similar device can be used in the gas vent line to maintain the desired back pressure on the gas vent device and system piping, while venting gas from the piping system.